A small thread on the Apple discussion boards talks about a problem one user is having with a small white rectangle appearing over the mouse pointer. It does not follow the pointer, but instead will just appear when the mouse is still and then may persist after the mouse has been moved.
This appears to be caused by recent versions of the Google Toolbar, so if you have this installed you might try removing it to see if the problem goes away. The toolbar is in beta testing, so you might send error reports and feedback to Google to help the development of the utility. Optionally, you might try removing preference files for the toolbar and reinstalling it. Being a beta release, Google may have updated the toolbar since you initially installed it, which might address this issue.
Any software has the chance to cause unforeseen problems, but this is especially true for beta and test versions, so generally if you see strange system behaviors the first step is to disable all third-party and test software you may be running. Most of the time that odd things like this happen, people end up having a myriad of menubar add-ons and other enhancements running in the background, any of which could be contributing to the cause for the problem.